Ophthalmic method and apparatus for laser surgery of the cornea

ABSTRACT

The invention contemplates a method for making a disposable element adapted for selective placement in the path of laser beam delivery to the cornea. The element carries a membrane of uniform thickness which is opaque to the laser-beam and which is subject to ablation when exposed to the laser beam. The central area of the uniform thickness membrane is then selectively exposed to the laser-beam so as to cause full depth removal at one locality in the central area and essentially zero depth removal at another area, so as to provide an article which, when interposed the cornea and an ablative laser beam, will, during a given laser-beam course of exposure will require greater or lesser time to locally ablate the membrane and thus permit laser-beam exposure past the membrane and into correspondingly localized ablating impingement with the cornea. Stated in other words, the article so manufactured will provide a varying spot size at the cornea on illumination with a laser-beam of uniform intensity profile.

RELATED CASES

This application is a continuation-in-part of my application Ser. No.07/146,045, filed Jan. 20, 1988, now U.S. Pat. No. 5,507,741, and saidapplication is a division of my earlier application Ser. No. 074,580filed Jul. 17, 1987 (now abandoned). Said earlier application is acontinuation-in-part of application Ser. No. 891,285, filed Jul. 31,1986 (now U.S. Pat. No. 4,732,148). Said application Ser. No. 891,285 isa continuation-in-part of application Ser. No. 778,801, filed Sep. 23,1985 (now abandoned); said Ser. No. 778,801 is a continuation-in-part ofapplication Ser. No. 742,225, filed Jun. 6, 1985 (now abandoned); andsaid Ser. No. 742,225 is a continuation-in-part of my originalapplication Ser. No. 552,983, filed Nov. 17, 1983, now abandoned, whichapplication Ser. No. 552,983 was continued-in-part as Ser. No. 748,358,filed Jun. 24, 1985 (now U.S. Pat. No. 4,665,913). The disclosures ofsaid applications and of other applications identified herein are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to that aspect of ophthalmic surgery which isconcerned with operations upon the external surface of the cornea.

Operations of the character indicated include corneal transplants andkeratotomies; such operations have traditionally required skilledmanipulation of a cutting instrument. But, however keen the cuttingedge, the mere entry of the edge into the surface of the corneanecessarily means a wedge-like lateral pressure against body cellsdisplaced by the entry, on both sides of the entry. Such lateralpressure is damaging to several layers of cells on both sides of theentry, to the extent impairing the ability of the wound to heal, andresulting in the formation of scar tissue.

My U.S. Pat No. 4,665,913 includes a background discussion of theeffects of various available wavelengths of laser radiation inophthalmic surgery and, in particular, surgery performed on the anteriorsurface of the cornea. It is explained that radiation at ultravioletwavelengths is desirable by reason of its high photon energy. Thisenergy is greatly effective on impact with tissue, in that molecules oftissue are decomposed on photon impact, resulting in tissue ablation byphotodecomposition. Molecules at the irradiated surface are broken intosmaller volatile fragments without heating the remaining substrate; themechanism of the ablation is photochemical, i.e., the direct breakdownof intra-molecular bonds. Photothermal and/or photocoagulation effectsare neither characteristic nor observable in ablations at ultravioletwavelengths, and cell damage adjacent the ablation is insignificant.

My later U.S. Pat. Nos. 4,669,466, 4,732,148, 4,773,414, 5,188,631 and5,219,343, illustratively deal with various concepts whereby laserradiation at ultraviolet wavelengths of 200-nm or less are controlled indelivery of laser radiation to the visually used area of the anteriorsurface of the cornea so as to penetrate the stroma and achieve apredeterminable volumetric removal of corneal tissue, thereby socorrectively changing the profile of the anterior surface as to reduce amyopia, or a hyperopia, or an astigmatic abnormality which existed priorto such laser surgery.

The present application deals with sculpturing per se, and it will beunderstood that the manipulative and preparatory and other operations,illustratively as described in U.S. Pat. No. 4,770,172 and in U.S. Pat.No. 4,773,414, are presently preferred in connection with thesculpturing method and means to be described herein.

The sculpturing technique of U.S. Pat. No. 4,732,148 may be brieflystated as involving corneal exposure to a laser beam of varying spotsize to achieve an ablated change in anterior-surface curvature in thevisually used central area of the cornea. Various means have beendescribed to achieve this result, and most conveniently the previouslydescribed means have involved microprocessor means to assure a givenexposure program in the course of which spot-size area varies aspredetermined to achieve a given profile change.

BRIEF STATEMENT OF THE INVENTION

It is an object of the invention to provide an improved method and meansfor variable-spot-size laser sculpture of the anterior surface of thecornea to achieve an optical improvement of the involved eye.

A specific object is to achieve the above object with simplifiedtechniques which do not require micro-processor control of spot-sizevariation.

Another specific object is to achieve the above objects by providing theophthalmic surgeon with an inventory of differently precharacterizeddisposable elements which can be selected from inventory in accordancewith the optical change to be achieved, so that, for example, thesurgeon can select, e.g., in diopter-related increments, for plus orminus spherical change and/or for plus or minor cylindrical change, orfor variously combined spherical and cylindrical curvature change, thechange he deems necessary to improve the optical performance of a giveneye.

The invention achieves the foregoing objects by providing an inventoryof disposable elements adapted for selective placement in the path oflaser-beam delivery of the cornea. Each of the disposable elementscarries a membrane which is opaque to the laser beam and which isdesignedly subject to ablation when exposed to the laser beam, thethickness of the membrane being a precharacterized function of localarea such that a given laser-beam course of exposure will requiregreater or lesser time to locally ablate the membrane and thus permitlaser-beam exposure past the membrane and into correspondingly localizedablating impingement with the cornea. Stated in other words,varying-spot size at the cornea is achieved by a time-release functionof the membrane, on a predetermined area-controlled basis that isdesigned to achieve the diopter change specifically identified with eachparticular selectable element.

DETAILED DESCRIPTION

The invention will be described in detail for illustrative embodiments,in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified view in vertical section showing means forsupporting a selected ablatable membrane element of the invention, foruse in laser surgery of a given cornea;

FIG. 1A is a fragmentary sectional view to illustrate a modification;

FIG. 2 is a graphical presentation, to differently enlarged andexaggerated abscissa and ordinate scales, to illustrate profiling ofablatable-membrane thickness to achieve a myopia-reducing result in thecontext of FIG. 1;

FIG. 3 is a simplified sectional view to illustrate the myopia-reducingsculptured-surface correction achieved with an ablatable-membranecharacteristic as in FIG. 2;

FIGS. 4 and 5 correspond to FIGS. 2 and 3, to illustrate thehyperopia-reducing sculptured-surface correction achieved with anablatable-membrane characteristic as in FIG. 4;

FIGS. 6 and 7 correspond to FIGS. 2 and 4, to illustrate a relationshipin the modification of FIG. 1A;

FIG. 8 is a simplified view similar to FIG. 1, to illustrate amodification whereby to enable preparation of an ablatable membranewhich incorporates precharacterized profiling to achieve a combinationof predetermined spherical and predetermined cylindrical correction, insubsequent laser surgery of a cornea.

In FIG. 1, the invention is shown in application to an eye-retainingdevice 10 which may be as described in U.S. Pat. No. 4,665,913. Device10 may thus be a hollow annulus, having a convergent axial-end wall 11of air-permeable material contoured to engage and retain an eye via ascleral-corneal region. A side-port connection 12 to a vacuum pumpenables retention of engagement of wall 11 to the cornea 14 to beoperated upon, and, optionally, the device 10 may be fixedly referencedto associated laser apparatus (by means not shown).

Preferably and suitably, the laser apparatus is an excimer laser,committed to pulsed ultraviolet radiation at 200 nanometers or less, asfor example provided at 193 nm by an argon-fluoride laser. Preferablyalso, the output beam of the laser is processed for shaping and forhomogeneity of flux distribution, for directional projection of ahomogenized coherent beam of circular section (e.g., of 5 or 6-mmdiameter) that is centrally aligned with the optical axis 15 of the eye.Means for processing the output of an excimer laser to produce such ahomogenized beam of circular section are described in detail incopending Telfair, et al. application, Ser. No. 009,724, filed Feb. 2,1987 and therefore need not now be further amplified, being identifiedby suitable legend in FIG. 1.

In accordance with the invention, an ablatable membrane 16 of suitablyprecharacterized thickness distribution is selectively positionable inthe path of laser beam projection on axis 15. Conveniently, the membrane16 is a coating on the upper surface of a plane-parallel supportingsubstrate 17 of material that is transparent to the laser beam; asuitable material for this substrate purpose is a synthetic fused silica(e.g., Suprasil, a commercial product of Heraeus-Amersil), or a fluoridecompound, such as calcium fluoride, magnesium fluoride, or bariumfluoride. In the form shown, substrate 17 is seated upon the inwardannular flange 18 of a ring member 19, which is adapted, via a dependingflange 20, for concentric engagement to the circular upper rim edge ofthe eye-retaining device 10.

The membrane 16 is ablatable under action of the laser beam at a ratewhich preferably (although not necessarily) corresponds with the rate oflaser-beam ablative penetration into the stroma of the cornea. Stated inother words, within the central circular area of laser surgery upon thecornea, the maximum thickness of the ablatable membrane 16 is such that,for the maximum extent of stroma penetration involved in a particularcorrective procedure, the surgical exposure can be terminated when theentire maximum thickness of exposed membrane is observed to have beenablatively removed. Suitable materials for the ablatable membrane 16include polyimide, Mylar and poly(ethylene terephthalate).

FIG. 2 provides illustration of the vertical-section thickness profilefor each of two selectively available elements, each of which isconfigured to effect a myopia reduction. For example, a profile 21 oflesser maximum thickness T₁ is seen to reduce from its maximum to zerothickness as a function of decreasing radius about the central axis, forthe case of a laser-beam section diameter of 5 mm. And a profile 22 ofgreater maximum thickness T₂ is also seen to reduce from its maximum tozero thickness as a function of decreasing radius about the centralaxis. The net effective difference between the two profiles is that itwill take greater time to reduce the involved central circular area ofmembrane 16 to a full 5-mm circular bore when membrane 16 ischaracterized by profile 22, than will be the case when membrane 16 ischaracterized by the profile 21; this fact translates into accomplishinga greater diopter reduction in cornea curvature for a membrane 16 ofthickness profile T₂ than is the case for a membrane 16 of thicknessprofile T₁. In both cases, laser-beam exposure should be terminated whenthe ablated membrane 16 is observed to attain a cylindrical bore, whichin the present illustrative case will be of 2.5-mm radius, achievingoptical-used curvature correction over a 5-mm diameter. In FIG. 3, thenewly achieved curvature correction is suggested by dashed line 23, inrelation to the more myopic original profile 24 of the cornea 14.

The thickness profiles 25-26 shown for membrane 16 in FIG. 4 arerespectively illustrative of what is needed in maximum membrane (16)thickness (a) to achieve hyperopia reduction of lesser degree, using thelesser maximum thickness T₁, and (b) to achieve hyperopia reduction ofgreater degree, using the greater maximum thickness T₂. In the case ofthe lesser thickness profile 25, thickness is greatest (T₁) at thecenter and is a decreasing function of radius to the point of zerothickness at the outer limit of optical correction (e.g., at 2.5-mmradius); in the case of the greater-thickness profile 26, thickness isalso greatest (T₂) at the center and is also a decreasing function ofradius to the point of zero thickness at the same outer limit of opticalcorrection. If the profiles 25-26 both terminated at the outer limit ofoptical correction, then the resulting sculpture of cornea 14 would becharacterized by a sharply defined outer cylindrical wall, but byutilizing the next 0.5 to 0.75-mm increment of radius to taper membranethickness back to maximum, i.e., radially outward of the outer limit ofoptical correction, it is possible to materially reduce the sharp-edgenature of such a wall. The ablation upon the cornea thus produces thehyperopia-reduced profile 27 (FIG. 5) out to the illustrative outerlimit of optical correction, and a gently beveled annulus 27' ofrelatively smooth transition to the remaining outer unexposed area ofthe cornea. It will be understood that to produce the "beveled"hyperopia-reducing result described in connection with FIGS. 4 and 5, itis necessary to adjust the circular section of the laser beam to aslightly larger diameter (e.g., 6.5 to 7-mm) than for the case ofmyopia-reduction (FIGS. 2 and 3), but such sectional-area selection isamong the capabilities of apparatus described in said Telfair, et al.application Ser. No. 009,724.

The ablatable-membrane technique described for the sphericallycorrective situations in FIGS. 2-3 and 4-5, respectively, will be seento be further applicable to achieve cylindrical correction needed forreducing an astigmatism. For example, if FIG. 2 is taken as depictingvertical-section profiles wherein the cylindrical axis of astigmatismcorrection is normal to and through axis 15 of FIG. 2 and wherein FIG. 2is no longer understood to depict a thickness profile of revolution, butrather a generally V-shaped channel profile extending along a diametralalignment with respect to ring 19, the described exposure course willachieve a cylindrical reduction of cornea curvature; and if ring 19 ispreset in rotation about axis 15 such that the cylindrical reduction isoriented to accord with the diagnosed orientation of the patient'sastigmatism, then the astigmatism can be reduced to a prescribeddiopter-reducing extent merely by correct selection of the maximumthickness of the thickness-characterized ablatable membrane 16. In FIG.1, an arrow indicator 28 on ring 19 can be brought by ring rotation intoregister against an azimuth scale 29 on device 10, to enable precisesetting of the orientation for an astigmatism-reducing procedure.

In the embodiments thus far described, the thickness-characterizedmembrane 16 is designed to achieve varying spot-size transmission ofstroma-ablating radiation, via a substrate which is transparent to theinvolved radiation. And in this situation, it is particularly convenientto embody the substrate and its ablatable membrane in a circular ringwhich is self-centering with respect to the optical size of the eye. Butrequisite transparent substrate materials may prove to be relativelyexpensive.

The fragmentary diagram of FIG. 1A indicates that substrate expense neednot be a problem, in the alternative event of using a plane mirror 30 asthe substrate which carries a thickness-characterized ablatable membrane16'. In FIG. 1A, mirror 30 is part of a ring element 31 which isinsertably located in a circular opening in supporting frame structure32, the latter being a fixed attachment to the laser housing. Mirror 30is shown inclined at 45 degrees to the incident laser beam so as to foldthe same for vertically downward surgical delivery to the cornea 14, tothe progressively varying spot-size extent determined by ablation ofmembrane 16' in the course of a given surgical procedure.

Before incidence with the membrane-coated surface of element 31, thelaser beam will be understood to be of homogeneously distributed fluxdensity across its circular section, which may again be of 5 or 6-mmdiameter for a myopia-reducing procedure. But at incidence with themembrane-coated surface, the area of incidence is an ellipse wherein theminor axis is horizontal; and the thickness profile at the minor axiswill be understood to be as depicted in FIG. 2, for a minor-axis extentof 5-mm, the extent shown in FIG. 2. On the other hand, the major-axisextent will be greater, being shown as 7.08-mm in FIG. 6; and the curves21'-22' of varying thicknesses, for the respective maximum thicknessesT₁, T₂ in the major-axis section plane of FIG. 6, are seen to follow theprofiles 21-22 of FIG. 2 on a correspondingly expanded scale. In anycase, the elliptical pattern over which membrane thickness ischaracterized for myopia reduction in the reflecting situation of FIG.1A will be understood to proceed from zero thickness at the center ofthe ellipse to maximum thickness T₁ (or T₂) at the perimeter of theellipse. Thus, in the course of a given myopia-reducing surgicalprocedure, the initially reflected beam will be a central spot ofcircular section, and this spot will progressively expand as thecharacterized membrane is progressively ablated, until the full circularsection of the incident laser beam is reflected into surgical incidencewith the cornea; at this point, the surgical procedure will haveaccomplished the prescribed myopia reduction at the anterior surface ofthe cornea.

Since it is important for operation of a reflecting system (as in FIG.1A) that the elliptical thickness pattern of ablatable membrane 16' becorrectively oriented such that the minor axis is horizontal, a keyingslot-and-stud engagement is schematically shown at 35 between a point onring 31 and a reference point on structure 32.

What has been said as to myopia reduction via ablation of a membrane 16'that has been precharacterized in minor-axis and major-axis sectionplanes according to FIGS. 2 and 6, respectively, can also be said forhyperopia reduction when membrane 16' has been precharacterized inminor-axis and major-axis section planes according to FIGS. 4 and 7,respectively. It will be recalled, however, from previous discussionthat a slightly larger circular-section laser beam is desired for thehyperopia-reducing procedure, and it can be clearly seen from FIG. 7that the ultimate sculpture of a bevel 27' is just as possible for thereflecting procedure of variable-spot size irradiation (FIG. 1A) as forthe transmitting procedure of FIG. 1.

The described invention will be seen to have achieved all stated objectsand to provide methods and means for more readily and economicallyperforming a laser-ablated recurvature of the cornea. The surgeon'sinventory of membrane-coated rings 19 (or 31) may be precharacterized bythicknesses and thickness profiles which can be labeled in terms wellunderstood by all ophthalmic surgeons, namely, in diopters of fractionsthereof, and qualified as to plus and minus spherical, with or withoutthe cylindrically correcting feature. The rings 19 may be prepared byapplying standardized coatings 16 of substrates 17, in thicknessesgraduated for successive increments of predetermined ultimate stromapenetrations to achieve given diopter changes; and the uniform-thicknessmembrane layer may be "cut" to its precharacterizing thickness profile,by laser ablation pursuant to the technique of one or more of theabove-noted related cases. For example, under micro-processor controland using the apparatus of FIGS. 8/9 or 15/16, respectively, of saidSer. No. 891,285, one may prepare for inventory a plurality of rings 19having membranes 16 with the FIG. 2 spherical or cylindrical correctingthickness profiles; and by using the apparatus of FIG. 30 of said U.S.Pat. No. 4,732,148, one may prepare for further inventory and pluralityof such rings 19 having membranes 16 with the FIG. 4 thickness profiles.

It should be noted that manufacture of thickness-characterizedreflection rings 31 for use in FIG. 1A is as simple and straightforwardas was the case for transmission rings 19 for use in FIG. 1.Specifically, using the same apparatus of said U.S. Pat. No. 4,732,148as described for characterizing membrane 16 for myopia reduction or forhyperopia reduction, the only difference is that a uniformly thick (T₁or T₂) membrane 16' should be inclined at 45 degrees to the incidenthomogeneous circular beam when precharacterizing the same. Suchinclination will enable automatic development of the described profile,with correct elliptical proportioning and orientation, using the keyingreference 35.

However, to manufacture rings 31 which are characterized for astigmatismcorrection, it is necessary, for use in the reflecting system of FIG.1A, to prepare a series of rings which have been precharacterized forastigmatism correction at each of a series of quantized increments ofazimuthal orientation. For the purpose of such manufacture, theastigmatism-reducing apparatus of FIGS. 15/16 of said U.S. Pat. No.4,732,148 is selectively adjustable for azimuth orientation, and is wellsuited to prepare such a series of rings 31.

The expression "zero thickness", as applied herein to describe one ofthe limits of precharacterized thickness profiling of membrane 16 (or16') is to be understood as a convenient way of stating (together withthe expression "maximum thickness") the range of thickness variationinvolved in precharacterization to achieve a given recurvature profileof the cornea. Thus, for certain purposes, it may be further convenientor desirable to avoid at truly zero thickness, as for example to providesuch added uniformly distributed thickness of membrane 16 (or 16')beneath the described profiling as to permit the surgeon to perform atesting of a given membrane 16 (or 16'), for example, prior to its usefor sculpture of a cornea. A test exposure of the homogenized beam tothe thus-characterized membrane could illustratively enable the surgeonto determine, as with his stop watch, the time required to expose to theextent of ablative decomposition of the added uniform-thicknessincrement, such time being visually observed to terminate upon initiallaser-beam emergence at the region of least-thickness of the membrane.When the surgeon determines this time, he has in effect calibrated hisinserted membrane for its rate of penetrating ablation of the membranematerial, thus establishing whether he has selected an element 19 (or31) with a membrane having the correct diopter-changing value which hehas prescribed, or whether he should achieve the prescription byselecting a different ring 19 (or 31) with a membrane designed for itsincrementally greater or lesser diopter-changing property.Alternatively, an element 19 (or 31) having a membrane 16 (or 16') thatis characterized by known uniform thickness may be a calibrating deviceforming part of the surgeon's inventory, in that his timing of hislaser's ability to ablate through the full membrane thickness of such anelement will enable him to immediately determine the current ablatingefficacy of his laser beam, thus enabling his appropriate allowance of adrop in ablating efficacy, merely by selecting for a given sculpturingprocedure an element 19 (31) having a thickness-characterized membrane16 (16') that was designed for a laser diopter-changing property, i.e.,of lesser maximum thickness.

FIG. 8 illustrates a further embodiment which utilizes a workpieceaccessory, namely, the plane-parallel plate 17 with uniform thicknessablatable-membrane layer 16 of FIG. 1, with plate 17 transparent toultraviolet radiation, or the reflective plate 30 with uniform-thicknessablatable-membrane layer 16' of FIG. 1A. Support means for a preferablycircular plate 17 (or plate 30) comprises an open-frame support member35 having a marginal ledge 36 for nested retention of a removablyinserted workpiece accessory, with the uniform-thickness ablatable layerin face-up relation to a beam of ultraviolet laser radiation, as from anexcimer laser 37. Support member 35 is tiltably mounted with respect toa base member 38, about a central transverse axis of frame 35. Aknurl-driven screw 39 provides selective tilt of the workpiece accessorywith respect to base member 38 (throughout an adjustment range α) andtherefore also with respect to incident laser radiation.

The laser 37 is indicated by legend to deliver a homogenizedcross-section of beamed ultraviolet radiation to a means 40 for varyingspot-size as a function of programmed time, pursuant to control bymicroprocessor means 41, to the end that a predetermined profile of theablatable layer is established for a given precharacterizing program. Ifthe workpiece accessory is set perpendicular to the delivered andprecharacterizing radiation, then the accessory will be manufactured asdescribed in connection with FIGS. 1 to 7; but if adjustment screw 39 isset for laser-beam incidence other than normal thereto, as for exampleat a preselected angle α, then a minor axis of elliptically definedablation of layer 16 will develop parallel to the pivot axis; at thesame time, a major axis of elliptically defined ablation of layer 16will develop perpendicular to the pivot axis, i.e., in the section planeof FIG. 8, and to the extent W suggested between arrows 42-42' in FIG.8.

It is a feature of the invention that if microprocessor 41 is programmedfor development of a spherical-curvature correction for the case of α=0,then for values of α greater than zero, a cylindrical component ofcurvature correction will be combined with spherical curvaturecorrection. The extent of requisite correction for each of thesecomponents will vary with individual prescription needs from one to thenext patient, and the proportions of cylindrical versus sphericalcombined profiling will depend upon experimentation.

In use, a workpiece accessory 16, 17 which has been prepared by ablationas described will be removable from the frame support 35 and will beknown to have a characteristic major-minor axis orientation which isrecognizable and which will therefore be insertable in the frame 19 ofFIG. 1, with correct angular orientation as dictated by the patient'sdiagnosed directional axis of astigmatism. A single period of laserexposure to a homogenized circulator beam (of at least minor-axisdiametrical extent) can then achieve the desired spherical/cylindricalcorrection, it being understood that the spaced arrows shown in FIG. 1for the legend "Homogenized Laser Beam" are schematically suggestive ofa desirable circular aperture or a circular beam section of at leastminor-axis diametral extent.

It is recommended that an inventory of accessory workpieces, ofdiffering but known ablatable thickness be kept on hand so thatdiffering diopters of curvature change can be more readily achieved,i.e., the greater the ablatable-membrane thickness, the greater thenumber of diopters of curvature change that can be profiled into a givenaccessory 16/17.

What is claimed is:
 1. The method of making a precharacterized accessoryfor use in the path of delivery of an ultraviolet laser beam to performa curvature-changing sculpture of the anterior surface of a cornea,wherein the accessory comprises a plane-parallel plate of substratematerial that is transparent to ultraviolet radiation, and an ablatablemembrane layer carried by said plate for expendable use in providing apredetermined ablative sculpture of the optically used central area ofthe anterior surface of the cornea, said membrane being opaque toultraviolet radiation and having a central circular area of varyingthickness which is a function of radius between central inner andradially outer limits of the circular area, the thickness ranging fromzero at one of said limits to maximum at the other of said limits, saidmembrane being of such maximum thickness within said circular area as torequire, for a given intensity of ultraviolet radiation, a predeterminedtotal exposure to achieve total ablation of said maximum thickness, saidpredetermined total exposure being that which is predetermined toachieve a given maximum stroma-penetration depth for a given sculpturingrecurvature of the anterior surface of a cornea exposed to ultravioletradiation via transmission through said plate; which method comprisespreparing said plate with a uniformly thick membrane layer on one planesurface thereof, the material of said membrane layer being opaque toultraviolet radiation, and exposing a central area of said layer toselective ultraviolet radiation and attendant ablativephotodecomposition in a volumetric removal of membrane material and withfull depth penetration at one locality within said central area, andessentially zero depth penetration at another locality within saidcentral area, such that a predetermined thickness profile is establishedbetween said localities, the ultraviolet radiation for such exposurebeing directed to said membrane layer with an orientation inclined inthe range 0 degrees to 45 degrees from a normal to said membrane layer,whereby the thickness profile resulting from such exposure may begenerally elliptical, with a major-axis profile which differs from aminor-axis profile.
 2. The method of claim 1, in which the ultravioletradiation is characterized by a beam having circular symmetry about acentral axis of incidence with said membrane layer, whereby toprecharacterized a profile component of spherical-correction properties,in combination with a profile component of cylindrical-correctionproperties.